The characteristics of a generally in accessible body through which a fluid flows can, in many cases, be determined by first adding to the fluid flow upstream of the body an injectate which modifies a property of the fluid, and then detecting changes in the modified property downstream. This technique is known in the field of medicine as thermodilution and generally involves injection of, for example, 10 cc's of cold saline through a catheter into a pulmonary artery. An incremental change in temperature produced by the cold saline is then detected downstream via a thermistor on the same catheter. The dead space in the injection lumen of a multi-lumen catheter is typically on the order of 1 cc, and accurate determinations require administration of volumes substantially larger the fluid resident in this dead space between injections. In addition, there is an uncertainty in the thermal loss involved in the injection process due in part to the transfer of heat between the lumen fluid and catheter environment, and in part to the warming of the fluid residing in the injection passageway between injections. To compensate for this loss, a large change in temperature, such as is only possible when a large volume of very cold saline is injected into the warmer bloodstream, is needed to swamp these small uncertainties. As a result, injectate volumes of approximately 10 cc's and temperatures of approximately 32.degree. F. (0.degree. C.) are generally required for acceptable accuracies.
While the injected fluid volume of the saline solution is innocuous for a single determination, it can become harmful to patients if repeated measurements are required. Many in the medical field have proposed automating the dilution measurement, so that it is repeated every few minutes with results of each measurement recorded. That sort of repetition, however, would require a substantial fluid administration in critically ill patients and require substantial refrigeration capability to cool the saline and maintain it at the required temperature. The former poses a health threat to the patient and the latter requires bulky, expensive equipment. These difficulties have frustrated the above-described attempts at automation.